The present invention relates to a projection display apparatus projecting images using light from a light source, and more particularly to a projection display apparatus that stores data, for example the data of a volatile memory, in a nonvolatile memory when an external power supply is forcibly cut off.
When a projector is used, a user can individually set some operating parameters such as brightness, color, contrast, and languages. Furthermore, the system of the projector updates and retains information such as a period of lighting, the number of lighting, and error information, by itself.
These data are tentatively stored in a RAM (Random Access Memory) that is a volatile memory. The data stored in the volatile memory are collectively stored in an EEPROM (Electrically Erasable Programmable Read Only Memory) or a FLASH-EEPROM that is a nonvolatile memory at regular time intervals. Such a control is performed since the access speed of the nonvolatile memory is slower than that of the volatile memory, and also the nonvolatile memory may have a limitation on the number of times data can be rewritten.
When an input power is cut off, the data stored in the volatile memory is deleted. However, the data stored in the volatile memory is periodically stored in the nonvolatile memory. Therefore, on the next occasion that input power is supplied to the system in the next time, the system will read the data from the nonvolatile memory and the original data will be restored. However, there is a problem when the input power is forced to be cut off before the updated data in the volatile memory are stored in the nonvolatile memory. In this case, the data that were not stored in the nonvolatile memory can not be restored on the next occasion that input power is supplied to the system in the next time.
Therefore, a conventional projector includes a secondary battery or an electric double layer capacitor that is capable of charging and discharging as a sub-power supply for a backup. Since the projector includes the sub-power supply for a backup, power is supplied from the sub-power supply for a backup when the system of the projector detects that the input power is cut off. The data in the volatile memory is stored in the nonvolatile memory while the power is supplied from the sub-power supply for a backup (See Japanese Patent Laid-Open No. 58-108932 or Japanese Patent No. 3113486).
However, a projection display apparatus including a sub-power supply for a backup increases in cost since it has the sub-power supply. In addition, the reliability as a projector decreases since the mounting area on a base is reduced and the number of devices for the sub-power supply increases.